Showing papers on "Momentum published in 2022"

Turbulent flow in curved channels

Abstract: Fully developed turbulent flow in channels with mild to strong longitudinal curvature is studied by direct numerical simulations. The Reynolds based on the bulk mean velocity and channel half-width . A log-law region is absent and a region with nearly constant mean angular momentum develops in the channel centre for strong curvatures. Spanwise and wall-normal velocity fluctuations are strongly amplified by curvature in the outer region of the concave channel side. Only near the walls, where curvature effects are relatively weak, do the mean velocity and velocity fluctuation profiles approximately collapse when scaled by wall units based on the local friction velocity. Budgets of the streamwise and wall-normal Reynolds-stress equations are presented and turbulence structures are investigated through visualizations and spectra. In the case with strongest curvature, the flow relaminarizes locally near the convex wall. On the concave channel side, large elongated streamwise vortices reminiscent of Taylor–Gortler vortices develop for all curvatures considered. The maximum in the premultiplied two-dimensional wall-normal energy spectrum and co-spectrum shifts towards larger scales with increasing curvature. The large scales substantially contribute to the wall-normal velocity fluctuations and momentum transport on the concave channel side.

Momentum relaxation effects in 2D-Xene field effect device structures

Abstract: We analyze the electric field driven topological field effect transition on 2D-xene materials with the addition of momentum relaxation effects, in order to account for dephasing processes. The topological field effect transition between the quantum spin Hall phase and the quantum valley Hall phase is analyzed in detail using the Keldysh non-equilibrium Green's function technique with the inclusion of momentum and phase relaxation, within the self-consistent Born approximation. Details of the transition with applied electric field are elucidated for the ON-OFF characteristics with emphasis on the transport properties along with the tomography of the current carrying edge states. We note that for moderate momentum relaxation, the current carrying quantum spin Hall edge states are still pristine and show moderate decay with propagation. To facilitate our analysis, we introduce two metrics in our calculations, the coherent transmission and the effective transmission. In elucidating the physics clearly, we show that the effective transmission, which is derived rigorously from the quantum mechanical current operator is indeed the right quantity to analyze topological stability against dephasing. Exploring further, we show that the insulating quantum valley Hall phase, as a result of dephasing carries band-tails which potentially activates parasitic OFF currents, thereby degrading the ON-OFF ratios. Our analysis sets the stage for realistic modeling of topological field effect devices for various applications, with the inclusion of scattering effects and analyzing their role in the optimization of the device performance.

The discrete Green's function paradigm for two-way coupled Euler-Lagrange simulation

Abstract: We outline a methodology for the simulation of two-way coupled particle-laden flows. The drag force that couples fluid and particle momentum depends on the undisturbed fluid velocity at the particle location, and this latter quantity requires modelling. We demonstrate that the undisturbed fluid velocity, in the low particle Reynolds number limit, can be related exactly to the discrete Green's function of the discrete Stokes equations. In addition to hydrodynamics, the method can be extended to other physics present in particle-laden flows such as heat transfer and electromagnetism. The discrete Green's functions for the Navier–Stokes equations are obtained at low particle Reynolds number in a two-plane channel geometry. We perform verification at different Reynolds numbers for a particle settling under gravity parallel to a plane wall, for different wall-normal separations. Compared with other point-particle schemes, the Stokesian discrete Green's function approach is the most robust at low particle Reynolds number, accurate at all wall-normal separations. To account for degradation in accuracy away from the wall at finite Reynolds number, we extend the present methodology to an Oseen-like discrete Green's function. The extended discrete Green's function method is found to be accurate within at all wall-normal separations for particle Reynolds numbers up to 24. The discrete Green's function approach is well suited to dilute systems with significant mass loading and this is highlighted by comparison against other Euler–Lagrange as well as particle-resolved simulations of gas–solid turbulent channel flow. Strong particle–turbulence coupling is observed in the form of turbulence modification and turbophoresis suppression, and these observations are placed in context of the different methods.

Modulation of fluid temperature fluctuations by particles in turbulence

Abstract: Modulation of fluid temperature fluctuations by particles due to thermal interaction in homogeneous isotropic turbulence is studied. For simplicity, only thermal coupling between the fluid and particles is considered, and momentum coupling is neglected. Application of the statistical theory used in cloud turbulence research leads to the prediction that modulation of the intensity of fluid temperature fluctuations by particles is expressed as a function of the Damkohler number, which is defined as the ratio of the turbulence large-eddy turnover time to the fluid thermal relaxation time. Direct numerical simulations are conducted for two-way thermal coupling between the fluid temperature field and point particles in homogeneous isotropic turbulence. The simulation results are shown to agree well with the theoretical predictions.

Computational Fluid Dynamics: Physical Law based Finite Volume Method

Abstract: After the Computational Heat Transfer (CHT) in the last three chapters, computational fluid dynamics is presented in this chapter and the next two chapters; for a Cartesian geometry problem. Furthermore, as compared to only energy conservation law for the CHT in the last three chapters, the mass and momentum conservation law are also considered for the CFD. Since the underlying molecular motion-based diffusion mechanism as well as bulk motion-based advection mechanism governs the momentum transport along with the energy transport, the earlier presented CHT development methodology for the heat transfer is generalized and extended here to include fluid dynamics. Note that the CFD here includes heat transfer along with the fluid dynamics, i.e., combined mass, momentum and energy transport phenomenon.

A Unified Framework for Governing Equations of Hydrologic Flows

Abstract: Laws of conservation of mass, momentum, and energy lead, respectively, to the equations of continuity, momentum, and energy, which are used to mathematically represent hydrologic flow syste...

Multiphase Flow Kinetic Theory, Constitutive Equations, and Experimental Validation

Abstract: The kinetic theory of gases is extended to particulate flow where the interaction between particles is not conserved. Granular temperature and the equation of state for the particle phase is developed. Granular temperature is defined as the average of the random kinetic energy, with the conversion factor of the Boltzmann constant. For dense flows, in addition to the kinetic and collisional stresses using the kinetic theory approach, a model account for the frictional stresses based on soil mechanics principles is presented. Gas/particle flows are inherently oscillatory and they manifest in non-homogeneous structures. Therefore, the drag force for different regimes of non-homogeneous gas/solid flow systems is discussed. Fluid/particle systems are composed of particles of different properties, in which transfer of momentum and segregation by size or density occurs during the flow, thus the kinetic theory was extended to modeling of multitype particle flow using the kinetic theory approach. Finally, heat and mass transfer equations for gas/solid flow systems are presented.

Introduction to Multiphase Flow Basic Equations

Abstract: Conservation equations for mass, momentum, and energy for multiphase flow are derived using the Reynolds transport theorem. The interfacial interactions between the phases are identified in mass, momentum, and energy equations for each phase. For multicomponent systems, conservation of species equations for the individual components in a phase are also presented.